In press-forming, when a thin sheet is press-molded by using a press mold having a curved projecting structure formed on the press mold, such as a bead or a projection having a small curvature, a defect in press-forming, such as a crack, necking (i.e., a constriction generated by the concentration of plastic deformation at a certain cross section of the thin sheet) or a wrinkle, may become problematic. Generally, such a crack, necking or wrinkle is very small, and therefore, it is very difficult to find in a manufacturing process.
Since a pressing force of a press machine or a reaction force due to deformation resistance of a workpiece to be pressed is applied to the mold at the time of press-forming, the mold may be elastically deformed. This elastic deformation is known as “mold strain”. The crack, necking or wrinkle, generated when the thin sheet is press-molded by using the press mold having the curved projecting structure as described above, is greatly influenced by occurrence of mold strain.
As a device for measuring the mold strain, Japanese Unexamined Patent Publication (Kokai) No. 5-337554 discloses a device for correcting a deflection of press brake having an upper beam with a punch and a lower beam with a die, the punch and the die being configured to move relative to each other so as to bend a workpiece between the punch and the die. This device includes: an upper beam strain sensor for detecting a strain of the upper beam; a plurality of lower beam strain sensors for detecting a strain of the lower beam, the sensors being arranged along the longitudinal direction of the lower beam; a plurality of actuators for applying a pressure force to an upper mold or a lower mold, the actuators being arranged along a machining line between the lower beam and the lower mold or between the upper beam and the upper mold; and a control means configured to stop the upper beam at a certain point while the pressurization is carried out, obtain a detection output by the upper beam strain sensor and the lower beam strain sensors when the upper beam is stopped, calculate strain amounts of the upper and lower beams based on the detection output, control the actuators based on the calculation such that the strain amounts become proper values, and restart the pressurization. This device is intended to produce a product having an uniform bending angle across the entire length of the product.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2004-249365 discloses a press-forming device having a punch; a die; a blank-holding die; a friction measuring means arranged between the die and the blank-holding die; and a blank-holding force adjusting means. This device is intended to generate a proper friction force regardless of variable factors, such as lubricity between the mold and a workpiece or the surface characteristics of the workpiece, and then provide a good product even when the material characteristics of the workpiece and/or the environment condition is varied.
In addition, some of the applicants of this application disclosed a press-forming device having a piezoelectric element (or a mold friction sensor) positioned near a shoulder of a die, for measuring a compressive or extensional strain in the orthogonal directions, in the 57-th Journal of Conference on Technology of Plasticity, (2006), pp. 165-166. By means of information from the mold friction sensor, a springback and/or a torsion regarding the shape of a product may be predictable.
Although Japanese Unexamined Patent Publication (Kokai) No. 5-337554 discloses an invention regarding a device having a measurement function of the mold strain, it is described that the beam strain sensors are arranged merely along the longitudinal direction of the beam for the press brake. In a press-forming process using a mold having a complicated shape relative to the beam for the press brake, in order to measure the mold strain of the mold with a high degree of accuracy, it is necessary to directly measure the mold strain by using a measurement means within the mold such as the punch, the die or the blank-holding die. In this connection, the invention as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-337554 is insufficient.
Further, in the invention of Japanese Unexamined Patent Publication (Kokai) No. 5-337554, the forming process is stopped before the forming is finished and the strain amounts of the upper and lower beams are detected while the forming process is stopped, and after that, the actuators are controlled such that the strain amounts become proper values and then the forming is restarted. However, in the press-forming, unlike the bending process of the press brake, if the press-forming is stopped during the process, friction between a workpiece and a tool becomes significantly different from the friction generated during the press-forming. Therefore, if the invention of Japanese Unexamined Patent Publication (Kokai) No. 5-337554 is applied to the press-forming, the mold strain will be different from that during the press-forming, whereby sufficient measurement accuracy cannot be obtained.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2004-249365 discloses a configuration having a strain measuring element sandwiched between a sheet and a blank-holding die fastened by a bolt or the like, for directly measuring friction. When a workpiece, sandwiched between the sheet and a die, is slidably moved, shear strain is generated at the strain measuring element and friction can then be measured. This configuration may be similar to the present application in using the strain measuring element. However, the measuring element of Japanese Unexamined Patent Publication (Kokai) No. 2004-249365 is used for measuring the friction by arranging a kind of structure on the blank-holding die or the die, therefore, the measuring element cannot directly measure the mold strain of the blank-holding die or the die.
In order to the mold strain with a high degree of accuracy, it is necessary to directly measure the mold strain of the punch, the die and/or the blank-holding die. In this connection, the invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2004-249365 is insufficient. On the other hand, the invention disclosed in 57-th Journal of Conference on Technology of Plasticity, (2006), pp. 165-166, includes a friction sensor positioned near the shoulder of the die. However, since the actual mold has a complicated shape, a shoulder of some molds cannot be clearly identified. Therefore, in the actual mold, it is difficult to determine where the strain sensor should be positioned, and the determination needs trial and error.